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Structure of benzene

Structure of benzene. Benzene (C 6 H 6 ). Hexane has a molecular formula of C 6 H 14 . Cyclohexane is C 6 H 12. Hexene has a formula of C 6 H 12 . Cyclohexene is C 6 H 10 . So obviously benzene must be a highly unsaturated molecule involving more than one multiple bond.

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Structure of benzene

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  1. Structure of benzene

  2. Benzene (C6H6) • Hexane has a molecular formula of C6H14. • Cyclohexane is C6H12. • Hexene has a formula of C6H12. • Cyclohexene is C6H10. • So obviously benzene must be a highlyunsaturated molecule involving more than one multiple bond.

  3. Kekule’s solution Double bond Single bond Double bond A ring linked by alternate bonds. Single bond Single bond Double bond

  4. Resonance structure of benzene Bonds are not fixed, but continually switch.

  5. But Kekule’s model does not fit the facts; 1) Benzene’s reactions are completely different from the alkenes. Alkenes, eg; cyclohexene, readily decolourise bromine. But benzene will not unless conditions are very vigorous.

  6. 2) Bond angles • If benzene consisted of alternate double and single bonds the angles in a benzene ring would be a mixture of 109.5 and 120. • In fact they are all 120O. • Producing a planar (flat) structure.

  7. 3) Energetics • Chlorine readily reacts with cyclohexene • ΔH= -183.7 kJ/mol. • Chlorine will react with benzene, but only if it is bubbled through boiling benzene in the presence of uv light. • If benzene had three double bonds; • ΔH= 3 x -183.7 kJ/mol = -551 kJ/mol. • In fact it is only -399.1 kJ/mol. • So benzene is more stable than predicted.

  8. Energy diagram + H2 + H2 + 3H2 -360 + 2H2 + 3H2 -240 real value -120 -152 kJmol-1 delocalisation energy Enthalpy (kJ mol-1) theoretical value -208 -120

  9. 3) Evidence from bond lengths In cyclohexane all C-C bonds are; 0.154 nm In cyclohexene the C-C bonds are 0.154nm, but the C=C bond is; 0.133 nm

  10. If benzene had the Kekule structure it should have… 0.133nm 0.154nm 3 C-C bonds of 0.154nm 0.154nm 0.133nm 3 C=C bonds of 0.133nm 0.154nm 0.133nm But in benzene all 6 C/C bonds have the same length…. 0.139nm Intermediate between C-C and C=C bonds.

  11. Each C atom forms two single bonds with other C atoms. C - C Forming a flat, hexagonal ring. C - H C - C NB All bond angles are 120o. And one bond to a H atom.

  12. Above and Below the plane. The fourth electron is left in p orbital.

  13. Electrons in the p orbitals overlap to form a delocalised ring…… Above And below the plane of the C-C ring So benzene is written as;

  14. Bonding in benzene H H C C above and below the plane of the atoms C C H H C C H H 6 electrons in a delocalised bond

  15. Physical properties of benzene. Benzene is a colourless liquid at rtp. BP = 353 K (hexane 354 K) MP = 279 K (hexane 178 K) The difference in MPs is due to the fact that flat benzene rings stack together well and are harder to seperate.

  16. Combustion They give smoky flames, as the high C:H ratii means that unburnt carbon is present. Arenes are flammable; C6H6 +7.5O2 → 6CO2 + 3H2O

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